function g=gobj_abs(x,a,b)

num=size(a,1);
% xi=randn(3,1);
id=unidrnd(num,1);
% g=2*(a(xi,:)*x-b(xi))*a(xi,:)'+sign(x);
g=2*sign(a(id,:)*x-b(id))*a(id,:)';
